1. Field of the Invention
The present invention relates generally to optical fibers, and in particular to an optical fiber having a large mode area and that operate as a single-mode optical fiber due to the attenuation of higher-order modes.
2. Technical Background
Optical fibers are the key component in fiber lasers. High-power, single-frequency fiber lasers have many different applications, including laser radar and imaging, frequency conversion for generating visible or ultra-violet (UV) light, as modules for spectral and coherent beam combining, as biomedical light sources, and in microfabrication.
It is generally desirable for a fiber laser to have as high an output power and as high a brightness as possible. However, as the output power increases, detrimental non-linear effects, such as stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS), increase. One way to increase a fiber laser's output power is to provide the optical fiber with a large mode area by decreasing the core relative refractive index and increasing the core diameter. However, the maximum output power of present day continuous-wave (CW) single-frequency fiber lasers or optical fiber amplifiers is limited to about several hundred watts due to stimulated Brillouin scattering (SBS), even when a large-mode-area (LMA) optical fiber with a core diameter of 20˜30 μm and a core numerical aperture (NA) of 0.05˜0.06 is used. Increasing the core diameter requires reducing the core NA to preserve diffraction-limited beam quality.
Although an ultra-low NA fiber with a NA of <0.03 can be formed using photonic crystal technology, such an optical fiber is very sensitive to fiber structural parameters, thus very difficult to manufacture.
Another option for overcoming nonlinear effects is to use a multimode optical fiber. To preserve diffraction-limited beam quality, the multimode optical fiber is coiled very tightly to provide tight bends that cause the high-order modes have much higher losses than the fundamental mode. Tight bending is typically defined as bend diameter of 50-150 times the outside diameter of a fiber. For highly multimode optical fibers (V number>4), however, the bending-induced loss difference between the fundamental and the high-order modes is so small that achieving diffraction-limited beam quality is virtually impossible.
For high-power fiber lasers, coiling the optical fiber to create a very small bend diameter is problematic. This is because most high-power fiber lasers require a large optical fiber diameter in order to couple sufficient pump light into the optical fiber. Tight bends in such optical fiber cause high pump-light loss and can lead to mechanical failure of the optical fiber. Furthermore, tight bending causes significant mode distortion and reduces the effective mode area.